Recruitment of resting stages may induce blooms of Microcystis at low N : P ratios

Some species of cyanobacteria form resting stages at the sediment surface when environmental conditions become unfavourable. As conditions turn more favourable, these resting stages hatch to the water phase, where the cells grow, reproduce, and sometimes form blooms. Since blooms of cyanobacteria have become an increasing threat to inland and brackish waters, it is important to assess the mechanisms and processes involved in the initiation of such blooms. One such mechanism is recruitment from the sediment surface. Potential factors regulating the recruitment of resting stages include variations in nutrient concentrations and ratios, as well as variations in grazing. To investigate how the recruitment of Microcystis responds to different levels of these factors, we performed an enclosure experiment (zooplankton abundances were regulated by predation from fish). We found that recruitment and growth were most pronounced at the second highest nutrient concentration (average concentrations were 498 mug l(-1) of dissolved nitrogen and 134 mug l(-1) of total phosphorus), while no direct response to different grazing levels was detected. We also found that resting stages can be important for initiating and sustaining blooms. The environmental conditions most important in regulating the recruitment rate from resting stages corresponded to the requirements of the plankton cells, namely high nutrient addition and low N:P ratio.